Op 15/03/2023 om 10:57 schreef scruel tao:
> The following code won’t be allowed in Java, but in python, it works fine:
> ```python
> class A:
> A = 3
>
> def __init__(self):
> print(self.A)
>
> def p(self):
> print(self.A)
> self.A += 1
>
>
> class B(A):
> def __init__(self):
> print(2)
> self.p()
> super().__init__()
>
>
> B()
> ```
>
> How can I understand this? Will it be a problem?
Important: __init__ is not a constructor, like you have for example in
C++. I don't know Java, but it seems plausible it works somewhat like
C++ in this regard. Python does it differently: when you create an
instance, the instance is fully created/constructed even before __init__
is called. __init__ is purely an initializer: you can use to initialize
the things you want to initialize.
Back to your example: it works because A is a class-level attribute,
which is initialized independently from __init__. If you make it an
instance attribute, like below, things stop working:
class A:
def __init__(self):
self.A = 3
print(self.A)
def p(self):
print(self.A)
self.A += 1
class B(A):
def __init__(self):
print(2)
self.p()
super().__init__()
B()
print(A.A)
That fails like this:
Traceback (most recent call last):
File ".code.tio", line 18, in <module>
B()
File ".code.tio", line 14, in __init__
self.p()
File ".code.tio", line 7, in p
print(self.A)
AttributeError: 'B' object has no attribute 'A'
That's because now A is indeed initialized in A.__init__, so it doesn't
exist before A.__init__ is called.
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comfort of opinion without the discomfort of thought."
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